1. Field of the Invention
The present invention is generally related to semiconductor manufacturing test measurement systems and, in particular, to the measurement of sheet resistance and junction leakage associated with p-n junctions and conductivity associated with metal films.
2. Description of the Related Art
The active elements of semiconductor devices within an integrated circuit (IC) are constructed in a succession of semiconductor layers, variously created using deposition and ion implantation process steps, and metal film depositions. These layers, particularly including implanted layers, are sensitive to fabrication-dependent process variables. In order to obtain appropriate electrical performance and process yield, implant fabricated layers must be highly uniform, both physically and electrically. Historically, implanted surface layers have been required to have a minimum and narrowly controlled range of sheet resistivity, ρs, across the surface of a wafer. Metal films are similarly required to have well-controlled film to semiconductor conductivity. Various test devices, including those known as four-point probe measurement devices have been used to measure the value and uniformity specifically of sheet resistivity.
The physical size of semiconductor devices has and will continue to shrink in an effort to improve integrated circuit speed and reduce power consumption. As fabricated active element areas shrink, the depth of the surface layer that contains the active drain and source implant regions, as well as other active regions, must also be decreased. Otherwise, operation of the active elements will become dominated by known short channel and other undesirable effects. A thin surface layer of less than about 500 angstroms can be created using low energy ion implantation followed by a spike or flash annealing, often on the order of one second or less. Such layers are conventionally known as ultra shallow junction (USJ) layers.
Precise production of the thin active element surface USJ layer is essential. Of particular concern is the level of junction leakage current through the ultra shallow junction. In conventional construction, a portion of fabricated drain regions will be defined by the USJ. Lower leakage levels, representing reduced device localized heat generation and overall power consumption, are desired. Given that the active surface layer implantation is performed early in the production of integrated circuits, accurate in-process measurement of both sheet resistivity and junction leakage is particularly desired to avoid the further production cost and time for circuits that will not perform acceptably as finally manufactured.
Another critical processing step occurs with the implantation of heavily doped local regions, commonly referred to as HALO implantations, to control and reduce short-channel effects between source and drain regions. Due to the high doping levels associated with HALO implantations, typically in the range of about 2×1018 to 2×1018 dopants/cm3, the leakage current at the associated ultra-shallow junction can be significantly increased. In-process control of the sheet resistivity and junction leakage properties is therefore highly desirable for HALO implanted USJ layers.
A recent method for in-process monitoring of USJ and HALO implanted layers utilizes a non-contact photo-voltaic propagation measurement to infer sheet resistance and leakage current is disclosed in U.S. Pat. No. 7,019,513, issued to Faifer et al. Non-contact is desirable for a number of reasons, including a high test-point rate and reduced measurement damage. Photo-voltaic inducement techniques do, however, have some disadvantages. The physical nature of photo-exited carriers will produce a forward bias across the p-n USJ layer to substrate junction in the range of tens of millivolts. In normal operation, this junction will be reverse biased typically in the range of several hundreds of millivolts to several volts. Test and operation regimes are therefore quite different. Further, in the absence of any actual surface contact, sheet resistivity and junction leakage must be indirectly inferred based on the propagation and attenuation of a indirectly sensed photo induced voltage. Consequently, measurement accuracy as well as the available measurement range of sheet resistance will be limited. The absence of any bias control makes measurement accuracy of junction leakage currents even less accurate. Even then, the photo-induced forward bias precludes any meaningful measurement of the junction leakage contribution due to tunning under reverse bias, as will be encountered in normal operating conditions.